Referring to FIGS. 1 and 2, typical modem telephone systems use a digital switch 4 to switch calls from one time-multiplexed channel onto several others. For example, time slots A and B from two different telephone calls may have been time-multiplexed onto a single input channel 1, but are destined for different output at channels 2 and 3. The digital switch completes a connection by transferring the signal portions A and B received from the particular time slots in the input channel 1 to specific time slots in the different output channels 2 and 3.
A digital switch generally performs its switching operations by temporarily storing data samples received on one of its input channels I0-IN and then distributing them to its output channels O0-ON. To keep up with the demands of switching between a certain number of input channels 6 and a certain number of input channels 8 at a particular rate, a switch should include a sufficient amount of computational resources, such as switching circuitry, data paths, and temporary storage. The switching rate is often a standard rate, but there are a variety of such rates in use. Voice-over-Internet systems, for example, often employ vocorders, which translate between so-called "full-rate" (64 kbps) audio telephone channels and "sub-rate" channels in which the audio data has been compressed to a fraction of the full rate.
A switch that supports sub-rate switching for a given number of channels is substantially more expensive than one that supports only full-rate switching. This is because resources within the switch are required on a per connection basis, and a switch supporting sub-rate switching must support far more connections. For example, a non-blocking switch supporting 1024 full-rate channels must support 1024 connections. But if the same switch also supported 8 kbps sub-rate channels, it would have to support 8096 connections.
Switches supporting such high connection densities are necessary in dedicated sub-rate systems, but they may be wasted in systems with mixed requirements. Although in very large systems it is possible to mix full-rate and sub-rate switches, smaller systems may present more difficult design choices. And the choices involved in systems tailored for specialized interface applications may be particularly problematic.